The invention relates to a structure control process comprising a field effect electron source.
A field effect electron source is usually in the form of several microprobes or studs, that are used as electron guns.
The microprobes, or studs may be aimed towards an acceleration anode lined with a luminescent material. This material emits visible light that can be used for display, in response to the electrons.
The microprobes, or studs, may also be aimed towards an anode equipped with a material capable of emitting x radiation in response to excitation of electrons.
Thus, the invention has applications for the manufacture of display devices for flat display screens and in the manufacture of X-ray tubes or any other device comprising an electron source comprising field effect elements (microprobes, studs, etc.).
Throughout the rest of the description, the example of a display screen using a microprobe source will be used in most cases for simplification reasons. However, and more generally, the invention is also applicable to any structure using a field effect electron source.
FIG. 1 shows a cross sectional view of a display screen with microprobes according to known art. For simplification reasons, only a few aligned microprobes have been shown. The screen consists of a cathode 1 which is a plane structure, laid out facing another plane structure forming the anode 2. The cathode 1 and the anode 2 are separated by a space in which the vacuum is formed. The cathode 1 comprises a glass substrate 3 on which the conducting level 4 is deposited in contact with the electron emitting probes 5. The conducting level 4 may be made in different ways. It comprises cathodic conductors usually associated with a resistive layer. The cathodic conductors may form various different geometries and particularly meshes. The conducting level 4 is covered by an insulating layer 6, for example made of silica, itself covered by a conducting layer 7. Holes 8 with a diameter of approximately 1 xcexcm have been formed through layers 6 and 7 down to conducting level 4 to deposit probes 5 on this conducting level. The conducting layer 7 is used as an extraction grid for electrons emitted by probes 5. The anode 2 comprises a usually transparent substrate 9 covered by a usually transparent electrode 10, on which one or several luminescent materials or luminophores 11 are deposited.
We will now describe the operation of this screen. The anode 2 is increased to a positive potential of a few hundred volts relative to the probes 5. A positive potential of a few tens of volts with respect to the probes 5 is applied to the extraction grid 7. Electrons are then detached at probes 5 and are attracted by the anode 2. The trajectories of electrons are included within a cone with a half-angle at the summit equal to xcex8 depending on the different parameters. The electron beam 12 is defocused by an amount that increases as the distance between the anode and the cathode increases. One of the methods of increasing the efficiency of the luminophores and therefore the brightness of the screen is to increase the acceleration voltage and therefore the potential difference between the anode and the cathode (typically between 1000 and 10000 V) which means further increasing the distance between the anode and the cathode in order to avoid the formation of an electric arc between these two electrodes.
If it is required to keep a good resolution on the anode, then the electron beam has to be refocused. This refocusing is typically done using an additional grid that may either be placed between the anode and the cathode (suspended grid) or placed on the cathode (integrated grid).
FIG. 2 illustrates the case in which the additional grid is located on the cathode. In order to make the drawing clearer, FIG. 2 repeats the example in FIG. 1 but for a single microprobe. An insulating layer 13 has been deposited on the extraction grid 7 and supports a metallic layer 14 acting as a focusing grid. Holes 15 with an appropriate diameter (typically between 8 and 10 xcexcm) and concentric with holes 8, have been etched in layers 13 and 14. The insulating layer 13 is used to electrically isolate the extraction grid 7 and the additional grid 14. The additional grid is polarized with respect to the cathode in order to make the electron beam 16 have the shape shown in FIG. 2.
In a flat color screen, the luminophores are deposited on the anode in the form of parallel redxe2x80x94greenxe2x80x94blue strips in sequence, etc. Color mixes must be avoided to achieve a good quality of the reproduced image. In order to achieve this, all electrons emitted by field effect elements to be aimed at a given color must arrive at the corresponding luminophore, and not adjacent luminophores. This result is achieved by the focusing phenomenon. Given the strip type structure of the luminophores, it is important that focusing should be done in the direction perpendicular to these strips in order to avoid color mixes. Similarly for a monochrome screen, it is useful to focus electrons on the luminophore, since this improves the screen resolution.
Moreover, for other structures comprising field effect sources, it is known that one or several other additional grids can be used, for example to protect the cathode from the electric field induced by the anode and thus avoid breakdown phenomena.
It is also known how to improve operation of microprobe screens by a particular addressing sequence that consists of planning regeneration phases during which the anode is brought to a sufficiently low potential so that it repulses electrons emitted by the microprobes.
This addressing sequence is described in the French patent entitled xe2x80x9cControl process for a flat display screenxe2x80x9d deposited on Jun. 8, 1995 by PIXTECH S.A., registration number 95 07017.
This addressing sequence has the advantage that it eliminates the phenomenon known as xe2x80x9ccolor driftxe2x80x9d. The color drift phenomenon corresponds to a change in color that occurs on the display screen when a uniform color corresponding to one of the three primary colors (red, green, blue) is displayed for a relatively long time varying from a few seconds to a few minutes.
The anode does not attract electrons during a regeneration phase. Luminophores are then not excited and the regenerated screen areas have no influence on the image formed.
In the case of high voltage screens, the acceleration voltage applied to the anode may reach several kilovolts. Fast switching of the anode potential to a sufficiently low potential with respect to the cathode potential, for example a few volts, is then difficult to achieve. The circuits necessary to apply this type of switching are complex, expensive and bulky.
The invention does not have the disadvantages mentioned above.
The invention relates to a control process for a structure with field effect elements, particularly for a cathode luminescent image display screen or for an X-ray tube, the structure comprising a cathode equipped with field effect elements, an anode, an extraction grid and an additional grid, the process comprising at least one phase in which electrons emitted by field effect elements are emitted to the anode and at least one regeneration phase during which the electrons emitted by the anode are not transmitted to the anode. The regeneration phase is implemented by application of an electron blocking potential on the additional grid, such that these electrons then return to the cathode. Return to the cathode means that electrons return to or near the cathode itself.
According to the invention, the regeneration phase is preferably independent of the emission phase concerning potential values applied to extraction grids and/or cathodic conductors during these phases. Thus, in the application to a display screen, the regeneration phase may be independent of the displayed image.
The invention also concerns a process for the display of images on a screen, characterized in that it uses a control process according to the invention as mentioned above.
The invention also relates to a process for the formation of images on an X-ray tube, characterized in that it implements a control process according to the invention like that mentioned above.
An advantage of the invention is that a high acceleration voltage (in other words potential difference between anode and cathode) can be used to attract electrons emitted by field effect elements while maintaining this voltage during regeneration phases. It is then no longer necessary to switch the anode potential, unlike the situation in prior art.
Other characteristics and advantages of the invention will become clear after reading a preferred embodiment with reference to the attached figures in which: